The present invention relates to a process for separating tungsten carbide from a titanium or titanium alloy substrate. Various aircraft components and engine components are today manufactured from titanium or titanium base alloys. Surfaces of such articles subject to wear are frequently coated with a wear resistant material such as tungsten carbide. In the recovery of titanium metal or titanium base alloy for reworking from scrap or salvage parts, or the resurfacing of otherwise sound parts it is necessary to remove the carbide coating prior to reworking the metal. Coatings of various kinds have been applied to titanium substrates for various purposes. These coatings have included plated material such as nickel and chromium, or scale as a result of oxidation of the surface due to high temperature heat treating, or the like.
For removing metal coatings from titanium or titanium alloy bases, it has been found that an electrolytic process can be used. This process utilizes as the electrolyte chromium trioxide (CrO.sub.3) dissolved in water. The workpiece is made the anode, and the cell is operated at a current density of between 100 and 250 amperes per square foot. The temperature of the electrolye is in the range of from 155.degree. to 185.degree. F. The results are improved by the inclusion of boric acid (H.sub.3 BO.sub.3). This is according to the process of Hall U.S. Pat. No. 2,316,579.
Titanium and titanium base alloys may be descaled of oxide coatings in a two step process disclosed by Covington U.S. Pat. No. 3,632,490. Two separate electrolytes are provided in separate tanks. In the first tank, the workpiece is the cathode of the cell, and in the second tank, the workpiece is the anode of the cell. In this case, the electrolyte is a solution of sodium dichromate and hydrofloric acid. The temperature of the electrolyte solutions is about 185.degree. F., and the voltage from 6 to 17 volts of 20 amperes. The current density in the cathodic cycle ranges from 100 to 1000 amperes per square foot, and in the anodic cycle should be between 80 and 1000 amperes per square foot. The time of exposure is approximately 5 minutes at 200 amperes per square foot.
In another process utilizing a titanium base metal, an electrolytic stripping process is taught by Cadieux U.S. Pat. No. 3,793,172. This process relates to the stripping of copper-nickel-chromium deposits from titanium. The stripping bath compositions consist of aqueous solutions of fluoboric acid (HBF.sub.4), phosphoric acid (H.sub.3 PO.sub.4) and water. In use the current density is about 16 amperes per square inch with an operating voltage of 7.5 volts DC. The stripping time was generally less than about 3 minutes.
In general, the prior art processes have depended upon deplating or mechanically loosening scale by electrolytically generated hydrogen gas using the workpiece as one of the electrodes in a predetermined electrolyte composition. In deplating, the materials which are deplated are elemental metals. In the present invention, the material being removed or stripped from the titanium or titanium alloy is tungsten carbide.